American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Community assembly and climate mismatch in Late-Quaternary eastern North American pollen assemblages”

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Clarke Knight, Jessica L. Blois, Benjamin Blonder, Marc Macias-Fauria, Alejandro Ordonez, and Jens-Christian Svenning (Feb 2020)

Plot twist! Over 21 ka, climate-assemblage mismatch increased during fast climate shifts and in high-latitude/tree-dominated areas

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Back to the future: lessons from the Quaternary for a modern world

What can the last 21 millennia in North America tell us about our future? Modern community ecology, which focuses on the here and now, is often disconnected from historical perspectives. However, important ecological processes – like vegetation response to climate change – manifest over a range of timespans, from decades to centuries, and millennia to deep-time. If the empirical richness of historical data could be more routinely applied to modern ecological concepts, ecologists could help forecast vegetation survival and persistence under future climate scenarios, as well as provide temporal context for modern concerns in community ecology. “We know that rapid environmental change defines our current world,” said lead author Clarke Knight, a PhD candidate at UC Berkeley in environmental science, “but by leveraging paleo data we can better understand past analogues that are applicable to modern situations.”

Knight teamed up with researchers from UC Merced, the University of Oxford, and Aarhus University to look at vegetation change in North America over the last 21,000 years using a large, publicly-available fossil pollen dataset. They investigated how well plant communities kept pace with climatic changes and if certain community qualities (like the amount of trees) could predict the level of matching with the past climate. They found, for example, that tree-dominated, high-latitude communities were often out of step with climate. Such findings help constrain predictions for community response to future climate change. And, more broadly, by working at the edges of modern and paleo sciences, researchers can utilize history and advance the science of ecology.


Plant community response to climate change ranges from synchronous tracking to strong mismatch. Explaining this variation in climate change response is critical for accurate global change modeling. Here we quantify how closely assemblages track changes in climate (match/mismatch) and how broadly climate niches are spread within assemblages (narrow/broad ecological tolerance, or ‘filtering’) using data for the last 21 ka for 531 eastern North American fossil pollen assemblages. Although climate matching has been strong over the last 21 millennia, mismatch increased in 30% of assemblages during the rapid climate shifts between 14.5 to 10 ka BP. Assemblage matching rebounded towards the present day in 10-20% of assemblages. Climate-assemblage mismatch was greater in tree-dominated and high-latitude assemblages, consistent with persisting populations, slower dispersal rates, and glacial retreat. In contrast, climate matching was greater for assemblages comprising taxa with higher median seed mass. Over half of the assemblages were climatically filtered at any given time, with peak filtering occurring at 8.5 ka BP for nearly 80% of assemblages. Thus, vegetation assemblages have highly variable rates of climate mismatch and filtering over millennial scales. These climate responses can be partially predicted by species’ traits and life histories. These findings help constrain predictions for plant community response to contemporary climate change.